1. Field of the Invention
The present invention relates generally to gypsum containing materials, and is also directed to improved methods for producing gypsum board and related materials.
2. Description of Related Technology
Today one of the most common manners of constructing walls and barriers includes the use of inorganic wallboard panels or sheets, such as gypsum wallboard, sometimes referred to as "wallboard," "drywall," or "plasterboard." Wallboard can be formulated for interior, exterior, and wet area (such as bathroom) applications. The use of wallboard, as opposed to conventional wet plaster methods, is often desirable because the installation of wallboard can be less costly and faster than installation of conventional plaster walls.
Walls and ceilings made with gypsum wallboard panels are conventionally constructed by securing, e.g., with nails or screws, the wallboard panels to structural members, for example vertically and horizontally oriented pieces of steel or wood such as "studs." Because wallboard is typically supplied in standard-sized sheets or panels, when forming a wall from the sheets or panels, there will generally be a number of joints between adjacent sheets. In most wallboard construction, these joints are filled and coated with wallboard tape and an adhesive material called joint compound so that the wall will have a monolithic finish similar to that obtained with conventional wet plaster methods. Various joint compounds are described in U.S. Pat. No. 5,653,797, assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference.
Generally, wallboard is conventionally produced by enclosing a core containing an aqueous slurry of calcium sulfate hemihydrate (e.g., calcined gypsum) between two large sheets of board cover paper. Gypsum wallboard is typically manufactured commercially by processes that are capable of operation under continuous high speed conditions, wherein the aqueous slurry of calcined gypsum and other ingredients are continuously deposited to form a core between two continuously-supplied moving sheets of cover paper. Various types of cover paper are known in the art.
The calcined gypsum forming the core between the two cover sheets is then allowed to set (react with water from the aqueous slurry). The continuously-produced board may then be cut into panels of a desired length (for example, eight feet). The formed board contains an excess of water because more water is required for working properties during gypsum slurry preparation than is necessary for hydration of the gypsum. The boards are then passed through a drying kiln in which excess water is removed and the gypsum is brought to a final dry state. After the core has set and is dried, the sandwich becomes a strong, rigid, fire-resistant building material called gypsum drywall.
Other methods for the production of gypsum wallboard are described, for example, in the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 21, pages 621-24 (Second Edition 1970) and Vol. 4, pages 618-19 (Fourth Edition 1992), the disclosures of which are hereby incorporated herein by reference.
A major ingredient of the gypsum wallboard core is calcium sulfate hemihydrate, commonly referred to as "stucco" or "Plaster of Paris." Stucco is commonly manufactured by drying, grinding, and calcining natural gypsum rock. The drying step of stucco manufacture includes passing crude gypsum rock through a rotary kiln to remove any free moisture accumulated in the rock from rain or snow, for example. The dried rock is then passed through a roller mill (a type of pulverizer), wherein the rock is ground to a desired fineness. The dried, ground gypsum can be referred to as "land plaster."
The calcination step is performed by heating the ground gypsum rock, and is described by the following chemical equation: EQU CaSO.sub.4.2H.sub.2 O+heat--&gt;CaSO.sub.4.11/2H.sub.2 O+.1/2H.sub.2 O.
This chemical equation shows that calcium sulfate dihydrate plus heat yields calcium sulfate hemihydrate (stucco) plus water vapor. This process is conducted in a "calciner," of which there are several types known in the art. Various methods of producing calcium sulfate hemihydrate are known in the art.
Uncalcined calcium sulfate (the land plaster) is the "stable" form of gypsum. However, calcined gypsum, or stucco, has the valuable property of being chemically reactive with water, and will "set" rather quickly when the two are mixed together. This setting reaction is a reversal of the above-described chemical reaction performed during the calcination step. The reaction proceeds according to the following equation: EQU CaSO.sub.4.1/2H.sub.2 O+11/2H.sub.2 O--&gt;CaSO.sub.4.2H.sub.2 O+heat.
In this reaction, the calcium sulfate hemihydrate is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction is generally dependent upon the type of calciner employed and the type of gypsum rock that is used, and can be controlled within certain limits by the use of additives such as accelerators and retarders.
In the hydration reaction, hemihydrate gypsum is mixed with water until a suspension is formed that is fluid and workable. The hemihydrate gypsum dissolves until it forms a saturated solution. This saturated solution of hemihydrate is supersaturated with respect to dihydrate gypsum, and so the latter crystallizes out of the solution at suitable nucleation sites. Finally, as the dihydrate gypsum precipitates, the solution is no longer saturated with hemihydrate gypsum, so the hemihydrate gypsum continues to dissolve. Thus the process continues to consume the hemihydrate gypsum. The reaction can be followed by measuring the heat evolved. Initially there is very little reaction and no rise in temperature. This time is referred to as the induction period. As the amount of dihydrate gypsum increases, the mass thickness increases and the material hardens (sets).
In order to facilitate the above reaction and/or provide beneficial properties to the final product, various additives may also be included in the core slurry. For example, starch, set accelerators and/or set retarders, preservatives, and fiberglass may be included.
As described above, the setting reaction for gypsum involves the reaction of calcium sulfate hemihydrate with water to form calcium sulfate dihydrate. The theoretical (stoichiometric) water content of the slurry required for the reaction of calcium sulfate hemihydrate is about 18.7 weight percent. However, a large amount of water is generally required to provide sufficient fluidity of the calcined gypsum slurry in order to obtain proper flow of the gypsum slurry in the manufacturing process. The amount of water required to provide proper fluidity depends upon various factors, such as the type of stucco, particle size distribution, the various phases of gypsum in the stucco, source, and the levels of above-described additives conventionally used in minor amounts.
Alpha-type stucco generally requires water usage of about 34 to about 45 cubic centimeters per 100 grams of calcined gypsum in order to form a readily pourable and flowable gypsum slurry. Beta-type stucco, on the other hand, typically has a water requirement of about 65 to about 75 cubic centimeters per 100 grams of calcined gypsum.
"Water reducing" additives may be included in order to improve the fluidity of the above-described gypsum slurry, while allowing use of reduced levels of water. Reduction in water usage brings reduced costs in the form of reduced water and energy demands, as less water will have to be removed during the drying step(s). Reduction of water usage also provides environmental benefits.
Various commercially-available fluidity-enhancing and/or water-reducing agents are known in the art for various applications. Typically the dispersing agent used in gypsum board manufacturing processes are calcium lignosulfonate, ammonium lignosulfonate, sodium lignosulfonate, and naphthalene sulfonate. Calcium lignosulfonate, sodium lignosulfonate, and ammonium lignosulfonate are believed to provide the ability to use reduced water levels, but they have the severe disadvantage of inhibiting the set of gypsum in the hydration reaction discussed above. Sodium lignosulfonate provides a weak paper-to-core interface in wallboard products. Ammonium lignosulfonate also has an objectionable odor due to the release of ammonia gas during the manufacturing process. The use of naphthalene sulfonate is limited, for example, due to its high cost. The use of condensation products of naphthalene sulfonic acid and formaldehyde is also known. See also U.S. Pat. No. 4,184,887, the disclosure of which is hereby incorporated herein by reference.
The use of the following additives in one or more applications is also known: anionic dispersing agents (such as alkylaryl sulfonates and lignin sulfonates) and higher molecular weight anionic condensation products (such as melamine formaldehyde modified with sulfite, as well as naphthalene sulfonate).
Water reducing agents are described in "The Gypsum Industry and Flue Gas Desulfurization (FGD) Gypsum Utilization: A Utility Guide," New York State Electric & Gas Corp. and ORTECH, pp. 3-38 (1994), the disclosure of which is hereby incorporated herein by reference.
The use of water reducing agents in another art, i.e., concrete, is described, for example, in "Water Reducing Chemical Admixtures," V. Dodson, Concrete Admixtures, Chapter 3, pp. 39-71 (1990), the disclosure of which is hereby incorporated herein by reference.
It would therefore be advantageous to provide a gypsum wallboard manufacturing process using a fluidity-enhancing additive that does not produce the set retarding effect suffered by some known manufacturing processes. Such wallboard also should be able to be manufactured through the use of conventional high-speed manufacturing apparatus, and not suffer from high cost or other negative side-effects such as detriment to long-term product performance.